CH Patent 700496 describes a shock resistant bearing formed by single crystal silicon and including a central portion and radial elastic arms connecting this central portion to a peripheral annular portion. The central portion includes a flared hole having the shape of a four-sided pyramid. First of all, it will be noted that the bottom of a four-sided hole is not optimum for supporting a pivot. As regards the making of a hole of this type, the above Patent provides anisotropic wet chemical etching. To achieve this, it is mentioned that the silicon substrate must be oriented properly to enable machining of the pyramid-shaped hole. Next, to machine the rest of the single-piece silicon part and in particular the elastic arms, the above Patent proposes using another machining technique, namely deep reactive ion etching (DRIE). This latter technique requires complex expensive plants which are different from those used for anisotropic wet chemical etching. Thus, the manufacturing cost of shock resistant bearings according to the teaching of the above Patent is relatively high. It will be noted that the use of two different techniques in different plants to machine the silicon parts does not arise from a propensity of the authors of CH Patent No 700496 to needlessly complicate the method of manufacturing silicon shock resistant bearings. It results in fact from a requirement arising from the properties of single crystal silicon. Indeed, the orientation of the silicon substrate required to obtain the flared pyramid-shaped hole cannot provide an elastic structure with arms having substantially vertical lateral walls, or the peripheral annular portion.
Generally, the present inventor has observed that silicon does not permit the machining of a structure with substantially vertical walls and exhibiting curvature by means of etching in an acid bath. Further, to obtain apertures in a single crystal silicon wafer with vertical walls, only specific silicon crystal orientations in the wafer are possible (incompatible with the orientation for obtaining pyramid-shaped holes). The possible directions for such vertical walls are limited and the vertical walls are only formed of plane surfaces.
WO Patent Application No 2009/060074 describes shock resistant bearings including a single-piece silicon part and a pierced stone associated therewith. This single-piece part defines an elastic structure and an endstone. It is formed in a silicon wafer using the well known techniques of photolithography and etching. This Patent document mentions that single-piece parts can be made of silicon or another preferably single crystal material easily machinable by photolithography and chemical etching techniques. No example is given other than silicon. As regards silicon, as mentioned above, although it is possible to obtain slots or apertures with vertical walls, designs are limited. In particular, it is not possible to obtain all the designs shown in the Figures of the above Patent document by chemical etching of a silicon crystal wafer. The teaching of the above Patent concerning the method of manufacturing shock resistant bearings made of single crystal material remains vague. Only the case of silicon is explicitly mentioned. The limits and drawbacks of a silicon crystal embodiment were set out in the discussion of CH Patent No 700496. Further, the meaning given to chemical etching here is not clear. In any event, it may be concluded that elastic structures such as those shown in the Figures are not made in an acid bath, but rather by deep reactive ionic etching as in CH Patent No 700496.
The Applicant of WO Patent Application No 2009/060074 also filed EP Patent Application No 2015147 (same priority dates). This latter document discloses a shock resistant bearing formed by a disc of single crystal material; said disc defining an elastic structure and a central portion with a blind hole intended to receive a balance pivot. In a variant, the elastic structure defines three interleaved spirals. The blind hole has a flat bottomed cylindrical shape, as shown in the Figures. It will be noted that the flat bottomed cylindrical shape is not optimum since the pivot moves and rubs against the cylindrical portion in an erratic manner, because the hole is wider than the portion of the pivot introduced therein. According to a main embodiment, that Patent document proposes using a single crystal silicon disc or wafer, which is machined using the well known techniques of photolithography (also called chemical processes).